The field of the invention is power controls and more specifically emergency stop and safety relay controls for use with power equipment.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
In many industrial systems, high levels of power are required by loads (e.g., manufacturing equipment, HVAC systems, etc.). Power relays are commonly employed to link and de-link power sources to and from loads, respectively. A typical power relay includes a coil and a plurality (e.g., four) of contact pairs or contactors where each contact contactor is either normally open (NO) or normally closed (NC) and changes its state (e.g., open or closed) when the coil is excited. In the case of a relay used with three phase power lines, the relay typically includes at least three NO contactors that close when an associated coil is energized and open when the coil is de-energized. In the case of an NO contactor, a relay spring usually biases the contactors into the normally open state. Typically the force applied by the spring to the contactors upon de-energization of the coil is on the order of one-fourth to one-half pound.
In addition to the components above, most high power control configurations also include several other components. To this end, a typical control configuration will include a start button and associated NO contactor, an emergency stop (ES) button and associated NC contactor and a fourth normally open power relay contactor (i.e., a fourth normally open contactor that opens and closes when the power relay is de-energized and energized, respectively) where the start and ES contactors are in series with the power relay coil and the fourth NO contactor is in parallel with the start contactor. In this case, to provide power to the load, the start button is pressed to close the NO start contactor thereby providing power through the ES contactor to the power relay coil which causes the power contactors in the power lines as well as the power contactor in parallel with the start contactor to close. When the start button is released, the parallel contactor remains closed so that the relay coil remains energized and the NO contactors remain closed.
If a problem occurs, a system operator can quickly cut off power to the load by simply pressing the ES button to open the ES contactor which cuts off power to the power relay coil and in turn, at least in theory, should open the NO relay contactors in the power lines as well as the NO contactor that is in parallel with the start button. Here, the force applied by the ES button to the ES contact pair is relatively large (e.g., on the order of 10 to 50 pounds, depending on the force applied by the system user when the ES button is pressed).
Unfortunately, as well known in the industry, despite cutting off power to the relay coil by pressing an ES button, under certain circumstances, the power relay contactors have been known to remain closed due to mechanical failure, heating/welding of contact pairs, residual magnetism within the relay structure, relay corrosion, frictional forces or a combination of the above. Hereinafter, in the interest of simplifying this explanation, the term “failed” will be used to refer to any NO contactor that remains closed when an associated relay coil is de-energized. Where any contactor in a relay fails, all of the NO contactors within a relay remain in the closed state. When the NO power line contactors fail, a load becomes uncontrollable as the system operator has no way to cut off power to the load.
To reduce the likelihood of uncontrollable loads, it has become common practice within the industry to design redundant power control configurations. For instance, one common redundant relay configuration includes two power relays where the relay coils are arranges in series with the NC ES contactor and the NO start contactor, a separate NO contactor from each of the two relays is arranged in series in each of the three power supply lines and an arrangement including series linked NO contactors from each of the relays is arranged in parallel with the NO start contactor. In this case, when a power relay contactor in the first relay fails (e.g., welds, sticks closed, etc.), in most cases the contactors in the second relay will remain operational and the load will remain controllable. Thus, even when one relay fails, when the NC ES button is pressed, the NO power line contactors in the second relay should open and cut off power to the load.
To better ensure redundancy, circuits have been developed that preclude providing power to a load after a relay fails until after the failure is eliminated via either manipulation of the relay or replacement of the relay. For instance, where corrosion causes a contactor to stick in the closed position, some times the contactor can be reopened by cycling through energizing and de-energizing cycles in an effort to overcome the binding effect of the corrosion. Where the spring force is insufficient to separate the NO relay contactors (e.g., in most cases where contacts weld together), the entire relay typically has to be replaced. While redundant relay designs and replacement relays are a solutions to the uncontrolled load and failure problems described above, unfortunately, these solutions are relatively expensive for several reasons. To this end, redundant relay designs require additional relay hardware which increases design and implementation costs. In addition, when the relay spring force fails to open NO contactors during energizing cycles and a relay has to be replaced, the replacement costs include loss of productivity due to down time of equipment linked to the power lines associated with the relay and maintenance costs (e.g., a system operators time) in addition to the cost of the replacement relay.
Moreover, in at least some cases conditions can occur wherein even redundant relay configurations fail to cut off load power when an ES button is pressed. For instance, when a large unexpected current surge passes through power lines it is possible for both series NO power line relay contactors in each power line to fail (e.g., weld) such that the ES button becomes effectively useless.
Therefore, it would be advantageous to have an inexpensive power control configuration wherein power to loads could be cut off despite the operational condition of line relays and where failed relays could be salvaged whenever possible despite contactor failure.